The present invention relates to a method for forming an interlayer insulating film and, more particularly, to a method for forming an interlayer insulating film having a low dielectric constant, which is necessary for a highly-integrated semiconductor device. A progress in high integration regarding the semiconductor device in recent years has resulted in a narrower interval between wiring lines. As the narrowed interval between the wiring lines causes an increase in capacitance between the wiring lines, a request has been made for formation of an interlayer insulating film, which has a low dielectric constant.
With recent progresses in high integration of an LSI device, the wiring line has been micronized and multilayered. There has also been an increase in capacitance between the wiring lines. Such an increase in capacitance has caused a great reduction in an operating speed. Thus, improvement in this regard has been strongly demanded. As one of improvement measures, a method for reducing capacitance between the wiring lines has been studied. This method uses an interlayer insulating film, which has a dielectric constant lower than that of SiO2 currently used for an interlayer insulating film.
Typical interlayer insulating films of low dielectric constants currently under study are {circle around (1)} an SiOF film, and {circle around (2)} an organic insulating film of a low dielectric constant. Description will now be made of these films.
{circle around (1)} SiOF Film
An SiOF film is formed by using source gas containing F and substituting Sixe2x80x94F bond for a portion of Sixe2x80x94O bond in SiO2. This SIOF film has a relative dielectric constant, which is monotonically reduced as concentration of F in the film increases.
For forming such SiOF films, several methods have been reported (see p.82 of monthly periodical xe2x80x9cSemiconductor Worldxe2x80x9d, February issue of 1996). Most promising among these methods is one for forming an SiOF film by using SiH4, O2, Ar and SiF4 as source gases, and by a high-density plasma enhanced CVD method (HDPCVD method). A relative dielectric constant of an SiOF film formed by this method is in a range of 3.1 to 4.0 (varies depending on F concentration in the film). This value is lower than a relative dielectric constant 4.0 of SiO2, which has conventionally been used for the interlayer insulating film.
{circle around (2)} Organic Insulating Film of Low Dielectric Constant
As an insulating film which has a lower dielectric constant (3.0 or lower) cared with the SiOF film, an organic insulating film of a low dielectric constant is now a focus of attention. Table 1 shows a few organic insulating films of low dielectric constants, which have been reported, and respective relative dielectric constants and thermal decomposition temperatures thereof.
However, the SiLOF film is disadvantageous in that an increase in concentration of F in the film leads to a reduction in moisture absorption resistance. The reduced moisture absorption resistance poses a serious problem, because a transistor characteristic and adhesion of an upper barrier metal layer are affected.
Peeling-off easily occurs in the organic insulating film of a low dielectric constant, because of bad adhesion with a silicon wafer or the SiO2 film. Furthermore, the organic insulating film is disadvantageous in that heat resistivity is low since a thermal decomposition temperature is around 400xc2x0 C. The disadvantage of low heat resistivity poses a problem for annealing a wafer at a high temperature.
It is an object of the present invention to provide a method for forming an interlayer insulating film having good moisture absorption resistance and heat resistivity and a low dielectric constant, a semiconductor device using the interlayer insulating film, and a semiconductor manufacturing apparatus for forming the interlayer insulating film.
According to the method for forming the interlayer insulating film according to the present invention, an underlying insulating film is formed on an object to be formed (a substrate), and then a porous SiO2 film is formed on the underlying insulating film. This porous SiO2 film is formed by three following methods.
(1) Chemical vapor deposition method using Si2H6 and O2 or Si3H8 and O2 as a reaction gas as illustrated in FIG. 1C.
The present inventor found that this method is used whereby Si2H6 and O2 or Si3H8 and O2 react with each other in a vapor phase and particulate SiO2 is formed in the vapor phase. Particulate SiO2 is deposited on an underlying insulating film 105. The surface of underlying insulating film 105 can not be densely filled with particulate SiO2 due to a shape of particulate SiO2. Thus, an SiO2 film 106 having many voids is formed on the underlying insulating film 105.
(2) Method in which plasma is intermittently or periodically generated in an atmosphere of SiH4 and O2 under a low pressure as illustrated in FIGS. 3C and 4
The present inventor found that the plasma is generated in the atmosphere of SiH4 and O2 under the low pressure whereby particulate SiO2 is formed in the vapor phase. The surface of the substrate can not be densely filled with particulate SiO2 due to the shape of particulate SiO2. Thus, the particulate SiO2 is deposited on the substrate, so that the porous SiO2 film is formed on the substrate.
The present inventor further found that the porous SiO2 film and a SiO2 film formed by typical low pressure chemical vapor deposition are laminated whereby the porous SiO2 film having a stable film quality is formed. The inventor invented the method in which the plasma is intermittently or periodically generated in the atmosphere of SiH4 and O2 under the low pressure, as the method for laminating these films. FIG. 4 shows an example of the plasma which is periodically generated. In this drawing, plasma enhanced chemical vapor deposition method is performed under the low pressure during a time period from T1 to T2. The low pressure chemical vapor deposition method is performed during the time period from T2 to T3.
(3) Method in which an organic film and the SiO2 film are alternately laminated and then the film is subjected to O (oxygen) plasma treatment as illustrated in FIGS. 6C and 6D
According to this method, a film 506 having the organic film and the SiO2 film, these films being alternately laminated, is first formed. Then, the film 506 is subjected to the O (oxygen) plasma treatment. The O (oxygen) plasma treatment is thus performed, whereby the organic film previously formed is selectively removed and thus the voids are created in an area in the film in which the organic film is formed. Thus, SiO2 alone remains in the film and many voids are created. That is, a porous SiO2 film 507 is formed.
(4) H (hydrogen) plasma treatment for the porous SiO2 film
The porous SiO2 film formed as described above has many voids in the film. Thus, a surface area of the porous SiO2 film is larger than the surface area of the SiO2 film having no void. Thus, the porous SiO2 film is prone to adsorb moisture in the air. As illustrated in FIGS. 1D, 3D and 6E, the porous SiO2 film is subjected to the H (hydrogen) plasma treatment. Thus, a dangling bond in an Sixe2x80x94O bond on the inner surface of the voids is replaced by an Sixe2x80x94H bond. Consequently, it is possible to prevent the moisture from being adsorbed on the inner surface of the voids.
(5) Formation of a cover insulating film
As illustrated in FIG. 1E, the porous SiO2 film is subjected to the H (hydrogen) plasma treatment, and then the porous SiO2 film is covered with a cover insulating film 109. Thus, it is possible to further prevent the moisture from being adsorbed on the surface of the porous SiO2 film 106.
Moreover, as illustrated in FIG. 8, the semiconductor manufacturing apparatus according to the present invention has control means for controlling flow rate control means for controlling a flow rate of the reaction gas and switching means for switching a high-frequency power applied to a chamber.
As illustrated in FIG. 4, this control means can alternately repeat the plasma enhanced chemical vapor deposition under the low pressure and the low pressure chemical vapor deposition in one chamber under the low pressure. That is, in FIG. 10, during the time period from T1 to T2, the reaction gases (SiH4, O2 and Ar) are introduced into the c and the high-frequency power is applied to the chamber, so that the plasma enhanced chemical vapor deposition under the low pressure is performed. During the time period from T2 to T3, the reaction gases (SiH4, O2 and Ar) are introduced into the chamber which the high-frequency power is not applied to, so that the low pressure chemical vapor deposition is performed.
Furthermore, as illustrated in FIG. 7, this control means can alternately repeat a process for forming the organic film by using the plasma enhanced chemical vapor deposition method and a process for forming the SiO2 film by using the plasma enhanced chemical vapor deposition method in one chamber which the high-frequency power is applied to. That is, in FIG. 7, during the time period from T1 to T2, the reaction gas (a CH compound) for forming the organic film is introduced into the chamber which the high-frequency power is applied to, so that the organic film is formed. During the time period from T2 to T3, the reaction gases (SiH4 and O2) for forming the SiO2 film are introduced into the chamber which the high-frequency power is applied to, so that the SiO2 film is formed.